Lithium ion batteries and ultracapacitors differ in their performance characteristics. Lithium ion batteries provide large specific energy while suffering from diminished power density, especially pulse power, temperature range, and operational and storage life. Meanwhile, ultracapacitors excel at providing high rate repetitive pulses across a wide temperature range with energy efficiencies near unity. Ultracapacitors have extremely high power densities and energy efficiencies due to their low internal resistance. Commercial ultracapacitors provide a specific energy of about 2 Wh/kg.
Rechargeable batteries involve several reactions throughout the structure, such as chemical processes that are highly reversible, but relatively slow (specifically bulk diffusion). Rechargeable batteries generally have a slight capacity loss due to parasitic side reactions that increase impedance and limit power and cycle life. Further, they often require waste heat management to maintain safety and cycle life.
Ultracapacitors (also known as supercapacitors or electric double-layer capacitors) are less prone to safety, thermal management and cycle life limitations that plague even the highest power rated batteries. For example, the majority of high power lithium ion batteries require thermal management systems to maintain their initial performance, prevent premature failure and potential catastrophic deflagration, resulting in severe safety concerns. Meanwhile, ultracapacitors can tolerate many more charge and discharge cycles, operate across a wider temperature range and are much safer than the most advanced battery chemistries.
Ultracapacitors can combine an electrostatic double layer capacitance with electrochemical pseudocapacitance and are about 10 to about 100 times the energy of an electrolytic capacitor. Ultracapacitors can access much more capacitance than an electrolytic capacitor due to fundamental differences in their internal structure. Instead of having two electrodes separated by an insulating layer like an electrolytic capacitor, an ultracapacitor employs a porous medium that significantly increases the serviceable surface area while not significantly increasing the charge separation between electrodes.
Electrostatic double layer capacitors store electrical energy by separation of charge in the Helmholtz double layer at the surface of a porous electrode and electrolytic solution. The charge separation distance in a double-layer is on the order of a few Ångstroms (from about 0.3 to about 0.8 nm) and is static in origin. The electrolyte serves as a conductive connection between the two active electrodes.
Electrochemical pseudocapacitors store electrical energy electrochemically through faradic redox reactions on the surface of suitable electrodes in an electrochemical capacitor. The reaction involves an electron charge transfer between the electrode and electrolyte along with a desolvated and absorbed ion. The faradic charge transfer is a rapid, highly reversible redox reaction that is very stable over time involving an absorption or intercalation process that does not involve the forming or breaking of chemical bonds. The faradic pseudocapacitance only occurs in tandem with an electrostatic double layer capacitance. The amount of pseudocapacitance depends on the electrode surface area, material and structure.
Pseudocapacitance is dependent upon the chemical affinity of electrode materials to the ions adsorbed on the electrode surface as well as on the structure and dimension of the electrode pores. The conductive electrode, often a high surface area carbon-based material (such as activated carbon) is commonly doped with transition-metal oxides to allow for pseudocapacitance. A number of transition metal oxides have been studied as alternative electrodes, using various techniques and baseline materials (substrates), such as, Co3O4 thin films prepared by sputtering or Co3O4 thin film on carbon fiber by hydrothermal synthesis, MnOx on carbon nanotubes by electrodeposition, NiO thin films by electrochemical precipitation, and TiO2 thin films by wet chemical method. Large pseudocapacitance, e.g., greater than about 1100 F/g (hydrothermal Co3O4 thin film on carbon fiber), was reported, along with good rate capability and cycle life for these materials. Alternatively, electrically conductive polymers such as polyaniline or derivatives of polythiophene have also been used to coat the electrode material. Pseudocapacitance may also originate from the structure, especially from the electrode pores. The pores in nano-structured carbons like carbide-derived carbons or carbon nano-tubes can be referred to as intercalated pores which can be entered by desolvated ions from the electrolyte solution. The occupation of these pores by de-solvated ions has been found to occur via faradic intercalations.